The present invention relates generally to an improved reference electrode system for monitoring certain parameters of blood, and more particularly to an improved coating for a reference electrode which is stable over extended periods of use, and thus capable of providing utility for an implantable device for the long-term and substantially continuous monitoring of blood parameters. Reference electrodes have been found useful in a number of blood monitoring applications, particularly in connection with determination of certain blood constituents, including pH, PCO.sub.2 and the like. In these applications reference electrodes may be utilized as condition responsive sensors to determine the presence of these certain constituents and particularly when continuous long term monitoring is either necessary or desirable. One application for the apparatus of the present invention is as a coated reference electrode for determining certain constituents of blood, and wherein the electrode is used in combination with an implantable cardiac pacer device. Such an arrangement utilizes the improved electrode for controlling one or more of the operating parameters of an implanted pacer while the pacer is being employed in connection with the long-term pacing of a patient's heart. In such an arrangement, operational parameters of the pacer will be determined in part by the condition responsive blood monitoring sensor of the present invention.
In the utilization of reference electrodes for blood monitoring applications, it is desirable, if not requisite, that the reference electrode be selected so as to be entirely passive to the tissue in the patients system. Adverse tissue reactions, such as inflammatory reactions and the like must be avoided, particularly when considering the utilization of the reference electrode for long term implantation applications. The coating of the present invention includes the utilization of a membrane having good electrical and mechanical properties while acting to physically separate the surface of the reference electrode from direct contact with the blood. This membrane must, of course, be sufficiently permeable to blood or at least to the constituents being measured so as to provide accurate, responsive, and reliable signals. Furthermore, the cross-sectional thickness of the film must be sufficiently limited so as to permit reasonable response times for the reference electrode, while being sufficient to provide surface isolation along with mechanical durability.
In connection with the present invention, the reference electrode is preferably silver/silver chloride, with such an electrode being desireable from the standpoint of reliability and stability within the hostile environment of the host body. However, silver chloride/silver when used alone has been found to be somewhat inflammatory toward human tissue. In accordance with the present invention, however, the silver chloride/silver reference electrodes are rendered essentially passive to tissue through encapsulation or coating of the reference electrode with the film prepared in accordance with the present invention. Also, the silver chloride portion of the reference electrode tends to be somewhat frangible, and the film coating of the present invention improves the durability and mechanical properties of the electrode. The coating material may find utility when used in combination with materials other than silver/silver chloride.